Problem with selecting faces vertices edges and texturing
Ok. I think I fixed it. What I did was delete the light (so I could select all and only get mesh parts). In object mode, I selected all, and right click &gt; join to make it all one piece (one mesh). I tried the CTRL+J method a couple of times first, so you might have to fiddle with it -  I don't know why it didn't work until I used the right click method. Anyway, once it was all joined into a mesh, I went to edit mode, then select all &gt; mesh &gt; clean up &gt; merge by distance - because there are &quot;seams&quot; between the edge pieces. I counted 1160 vertices removed. After that, I had to UV unwrap again because there were overlaps, but I think it's good to go. If, in the future, you need it to be more pieces, you can always separate it later.
I'll upload a copy of the file I modified for reference.
Here it is - 
Hope this helps.
EDIT - Here is your file &quot;cleaned up&quot; it still is not perfect - I did a quick and messy job on the UV unwrap just so I could get the textures to look decent - you may have to redo them. Also, many of the floors are still n-gons, this might not matter, but if you want to triangulate for a game engine later, you might want to make sure it's all quads first to be sure there a re no issues. I separated what &quot;chunks&quot; of floor looked appropriate, but obviousely, you will want to divide some of them on your own for texturing reasons - i'll leave that to you. I also have to remove your texture in order to pack the ones I used, you'll have to replace it.
Here is the (cleaned up) file.  Hope this finally works for you.